BREAKING(path/unstable): move unstable overload of dirname to unstable-dirname

[kt3k]

part of #5920

This PR moves the 2nd overload of dirname functions to ./unstable-dirname.

Unstable versions of dirname functions are only available from unstable-dirname, posix/unstable-dirname, or windows/unstable-dirname.

If this looks fine, I'll work on the rest of items in @std/path

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.32%. Comparing base (b900e72) to head (0c53a61).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5954   +/-   ##
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  Coverage   96.31%   96.32%           
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  Files         483      484    +1     
  Lines       39402    39403    +1     
  Branches     5839     5837    -2     
=======================================
+ Hits        37952    37956    +4     
+ Misses       1408     1405    -3     
  Partials       42       42           

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[iuioiua]

Nicely done

[iuioiua]

Wait, we should suggest to use the unstable APIs in the stable API's documentation.

[iuioiua]

This suggests that the unstable API will one day become the stable API. Ditto for the other functions.

Suggested change

	* use `dirname` from `@std/path/unstable-dirname`.
	* use the new version of `dirname` from `@std/path/unstable-dirname`.

[iuioiua]

Nice. We really should document our new convention for unstable modules somewhere.